In the art of loudspeaker systems it is desirable to obtain the extended low frequency response. In addition, it is generally desirable to minimize the size of the loudspeaker enclosure, for example to reduce cost and allow for more flexible placement. These two goals are often in opposition, and it is well known that obtaining extended low frequency response typically requires large, floor standing speakers with significant internal volumes, and/or large diameter woofers. Both options require tradeoffs in terms of efficiency, cost and flexibility of use, with large speakers typically being less efficient, costing more, and being less flexible in terms of placement in a listener's home.
There are a number of industry standard loudspeaker design approaches that have been used for many decades to achieve extended low frequency response. They generally fall into the categories of acoustic suspension, bass reflex, horn, and labyrinth or transmission line. The basic sealed enclosure or ‘acoustic suspension’ system, while the simplest of the devices, has significant limitations, typically including low efficiency and requiring very large driver diaphragm area and excursion capability to achieve reasonable outputs at low frequencies.
Bass reflex, or vented systems can increase efficiency by 3 dB or extend the −3 dB low frequency cutoff by approximately a half octave, or reduce enclosure size and achieve the same output at the same low frequency as a similarly sized sealed enclosure. These improvements are offset by problems with enclosure standing wave and pipe resonances exiting the vent, and for standard, maximally flat alignments, the systems are substantially ineffective at extending response below the free-air resonance of the transducer. in addition, vented design have problems with extreme diaphragm excursions below the cut-off frequency, reducing maximum output or requiring high pass filters to protect the woofer.
Transmission lines pass the acoustic output throughout an elongated labyrinth having a line length typically being ¼wavelength of the lowest usable frequency range; achieving extended low frequency response thus requires substantially increasing the size of the enclosure. In addition, the transmission lines utilize substantial damping material throughout the line length, which further reduces efficiency.
Existing expansion horns are known for high efficiency, but to achieve their potential they must have high expansion rates and horn lengths that correspond to approximately ¼ to ½ wavelength of the cut-off frequency. Again, this requirement results in very large sizes for a given low frequency capability.
Variations of the horn and pipe structure have been used to create tuned pipes, which also depend on a ¼wave pipe length at a lowest tuning frequency and cut-off frequency. These systems also suffer in having uneven frequency response and poor group delay, due to uncontrolled resonances in the transmission line.